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Breakthrough in Accelerator Physics: Crab
cavities are operated successfully for the first time.
Tsukuba, Japan, 3 July 2007: A team of accelerator physicists at the
High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, has
achieved effective head-on collisions of electrons and positrons using new
devices called "crab cavities" during the spring operation period of the KEKB
accelerator. This success will pave the way to increase KEKB's luminosity, which
is already the world highest, to an unprecedented level.
At KEKB, beams of electrons and positrons collide at nearly the speed of light
and annihilate into a state of pure energy. These collisions produce pairs of
particles called B mesons and anti-B mesons. These mesons have several thousand
decay modes; therefore, it is essential for the experiment to have as many pairs
as possible. The rate of collisions, called luminsity, is the most critical
parameter for the successful operation of KEKB.
The electron and positron beam bunches cross at an angle of 1.3 degrees at KEKB.
This non-zero crossing angle is one of the novel design features of KEKB,
providing effective beam separation at the collision point without a high level
of background noise in the detector. To boost the luminosity further, it is
necessary to tilt the bunches of electrons and positrons for effective head-on
collisions while retaining the crossing angle.
To accomplish this goal, the KEK researchers built several superconducting
radio-frequency cavities or "crab cavities" that tilt each bunch sideways
(somewhat akin to the way a crab walks) so that they collide head-on at the
interaction point. At low currents, the crab cavities achieved a tune-shift
comparable to the world record. Tune-shift is a quantity proportional to the
luminosity divided by the product of beam currents and is a measure of
luminosity potential. Recently, at high beam currents (1300 mA in the low energy
positron beam and 700 mA in the high energy electron beam), they were able to
operate at a luminosity above 1034/cm2/sec. These results from the first round
of commissioning demonstrate the potential of the crab cavities, which according
to simulations may eventually improve the luminosity by a factor of two. More
commissioning runs and R&D will enable further increase of the performance.
The concept of crab cavities was first suggested almost 30 years ago by R.
Palmer for linear electron-positron colliders. In 1989, K. Oide and K. Yokoya
proposed the use of crab cavities in storage rings. This was followed by designs
and prototype models of the crab cavity by K. Akai as part of a collaboration
between the KEK and Cornell laboratories around 1992. Detailed engineering and
prototyping were done at KEKB by K. Hosoyama's team. The first full-size
cavities were developed and installed in January 2007. Commissioning at KEKB
started in February 2007 and continued until the end of June.
The KEKB collider is used by researchers for studies of matter-antimatter
asymmetry with beauty (b) quarks and searches for new physics by the Belle
experiment, a collaborative international effort of scientists from universities
and research institutes. The Belle experiment has reported a number of
fundamental discoveries concerning matter-antimatter asymmetry and rare decays.
For the future, a super B Factory that will use crab cavities for higher
luminosities is under discussion. Such a machine has the potential to discover
physics beyond the Standard Model in rare decays. Crab cavities will also play a
role in achieving high luminosity at other machines with a crossing angle,
including the International Linear Collider (ILC), upgrades of the Large Hadron
Collider (LHC) at CERN as well as future synchrotron light sources.
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